1. Field of the Invention
This invention relates to an optical disk apparatus having a controller which carries out a seek operation and a tracking control operation for a light beam irradiated from an optical head while the light beam is moved along a surface of an optical disk and positioned on a selected track of the optical disk.
2. Description of the Related Art
A structure of a conventional optical disk apparatus will be explained with reference to FIG. 1.
In an optical disk apparatus as shown in FIG. 1, data are written onto a recording track on a recording medium such as an optical disk 1, by a beam spot focused on the optical disk 1, and the data are read out from light reflected from the optical disk 1. The optical disk 1 is rotated by a motor 1a around its center axle 1b. An optical head 2 which is driven by a motor (not shown) travels coarsely along a radial direction of the optical disk 1. The motor (not shown) is controlled by a carriage controller 5. The optical head 2 includes a semiconductor laser 24 as a light source to emit a light beam, an object lens 20, a lens 25, and a polarizing beam splitter 23. The light beam emitted from the semiconductor laser 24 is passed through the lens 25 and the polarizing beam splitter 23, and focused by the object lens 20 as a beam spot BS projected onto the optical disk 1. The optical head 2 also has a track actuator 21, e.g. tracking coil, a focus actuator 22, such as a focusing coil, a 4-division light receiver 26 and a lens 27. A light reflected from the optical disk 1 is input into the 4-division light receiver 26 through the object lens 20, the polarizing beam splitter 23, and the lens 27. The track actuator 21 is provided to move the object lens 20 along the radial direction of the optical disk 1 to track the beam spot BS on a selected track. The focus actuator 22 is provided to move the object lens 20 along the axial direction of the object lens 20 to focus the beam spot BS on the optical disk 1. The track actuator 21 is controlled by a fine tracking controller 3 which generates a tracking error signal in accordance with the RF signal output from the light receiver 26 and sends the tracking error signal to the track actuator 21 to adjust a position of the beam spot BS to the selected track to be followed.
The focus actuator 22 is controlled by a focus servo controller 4 which generates a focusing error signal in accordance with the RF signal output from the light receiver 26 and sends the focusing error signal to the focus actuator 22 to adjust a focusing point of the beam spot BS to a surface of the optical disk 1. The principle of the fine tracking control and the focusing control are well known, so an explanation of these will be omitted.
In the structure described above, in order to move the beam spot BS through many of the tracks to a target track, a so-called track-jumping method is used to move the optical head 2 by driving an actuator, such as a motor (not shown), along the orthogonal directions of the tracks for a long stroke, for example 120 mm. The actuator is controlled by a main controller using a positioner seek and tracking control. In the positioner seek operation, the main controller produces a target light beam velocity pattern including an acceleration term, a constant velocity term, and a deceleration term, and controls an actual light beam velocity detected by a sensor to be consistent with a target light beam velocity produced by the main controller, in accordance with an error value between the actual light beam velocity and the target light beam velocity.
When the light beam spot reaches the target track in the positioner seek operation, the main controller uses a tracking control operation in addition to the positioner seek operation, and positions the light beam spot on the selected target track. In the method described, however, since the tracking control is used after the light beam spot has reached the target track and the tracking actuator 21 is in a standstill at that time, if the optical disk 1 rotates with an eccentric movement, and the tracking actuator 21 cannot follow the eccentric movement, it is necessary for the main controller to re-seek to the target track. Therefore the track access time of the prior art optical disk apparatus becomes slow.
In order to resolve this problem, there is known a track-jumping method by which a main controller controls an optical disk apparatus to move a tracking actuator toward the target track while moving the optical head with the positioner seek operation. That is, when the light beam velocity is lower than a predetermined velocity, a main controller stops the control of the positioner seek operation and begins a lens seek operation in which the tracking actuator is moved under control of the main controller toward the target track and the optical head is moved to follow the tracking actuator toward the target track in accordance with an output signal of a sensor which detects the movement of the tracking actuator.
In this method, because a velocity servo control of the light beam would be used in the positioner seek operation and lens seek operation, different methods must be used for detecting a light beam velocity between the position seek operation and lens seek operation, so the main controller cannot control the tracking actuator after the velocity servo control of the light beam by the main controller is changed from the positioner seek operation to the lens seek operation.